The scientists were able to predict blue or brown eye colour with unprecedented accuracy. Credit: Corbis

Police might soon be able to tell the eye colour of criminals from DNA recovered from crime scenes, thanks to a new genetic study.

A team of scientists led by Manfred Kayser, of Erasmus University Medical Centre in Rotterdam, has developed a method that predicts eye colour with unprecedented accuracy.

The method could also be used on ancient DNA to profile early modern humans or even Neanderthals. And as eye colour is determined by many genes, the work also raises hopes for predictive genetic tests for diseases that are influenced by numerous genes, such as heart disease.

Kayser's team looked at DNA from more than 6,000, Dutch Europeans from Rotterdam. About 68% of these people had blue eyes, whereas around 23% had brown eyes.

The researchers focused on eight genes known to be associated with eye colour. These genes code for proteins involved in the production or distribution of the iris, skin, and the hair pigments eumelanin and pheomelanin. The strongest effect on eye colour is determined by two adjacent genes, OCA2 and HERC2, on chromosome 15, but these don't paint the full picture.

Kayser's team zeroed in on 37 sites of genetic variation in the eight genes. These sites — called single nucleotide polymorphisms (SNPs) — are points where one of the four letters in the DNA code of chemical bases is substituted for another.

Colour code

The scientists used powerful mathematical algorithms to find the smallest subset of these SNPs that could be used to predict eye colour accurately and found six that predicted blue and brown eye colour with just over 90% accuracy. The team report their work in Current Biology1.

The accuracy of the test was lower — just over 70% — for eyes of other colours. Most people have brown or blue eyes, with blue the dominant colour for those with roots in Northern Europe but brown predominating among those with ancestors from Southern Europe or from elsewhere in the world.

So far, the test has been shown to be accurate only for Dutch people of European ancestry. Further work would be needed before the test could be used in other countries.

The best predictive tests to date have only had an accuracy of around 80%, Kayser says. "That's not bad but it is not accurate enough for forensics," he says. The Netherlands Forensic Institute plans to use the new method in forensic case work within a year.

Dutch laws allow use of DNA to predict eye colour. But Kayser notes that many countries would have to change their legislation to allow the test to be used in forensic applications.

Forensic geneticist Peter de Knijff, of the Netherland's Leiden University Medical Center, says that the results will spur the race to develop more reliable genetic tests for hair and skin colour.

"Those three — eye colour, hair colour, skin colour — combined with a rather accurate global test for the inference of the geographical origin of an individual will probably be built into a general package that you could use to get information about an unknown suspect," he says.

The method could, in theory, also be used to predict the eye colour of a fetus — fuelling fears about 'designer' babies. "That would have different ethical and legal implications," Kayser stresses.